MAC is defined as the minimum alveolar concentration at a steady state of inhaled anaesthetic at 1 atmospheric pressure that prevents movement (e.g. withdrawal) in response to a standard surgical midline incision in 50% of a test population. It reflects the actions of an inhalation agent on spinal cord-mediated reflexes by measuring somatic responses and is not necessarily a surrogate for lack of awareness.
The minimum alveolar concentration (MAC) is widely known to decrease as age increases. Bispectral index (BIS) values provide a convenient method to avoid overdosage or underdosage of anaesthesia, especially in the elderly population. We assume that BIS values correlate with MAC values, provided we used age-corrected MAC for each of our patients, calculated using Mapleson formula. Elective surgeries requiring supraglottic airway insertion and belonging to American Society of Anesthesiologists physical status I and II were considered. We observed BIS values of each of our patients who were maintained on age adjusted MAC of desflurane. We then divided the study population into two groups: 18-40 years and 41-60 years. All 112 patients finished the study. BIS value at the end of 15 minutes in the age group 18-40 years was 43.2 with a standard deviation of 7.5, as compared in the age group of 41-60 years where it was 42.5 with a standard deviation of 7.4. In both the groups, BIS values remained within the normal range for general anaesthesia. The present study showed that there is positive correlation between BIS and MAC provided, age adjusted MAC value is calculated for all patients. It can hence, avoid the deleterious effects of both over and under administration of general anaesthesia.
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2022: 6.012
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